In an alarm or security installation, it is often required to detect the status of electrical switches which are employed in door and window sensors or other alarm sensors. Apparatus is known for providing coded signals which represent the open and closed states of a switch, examples being shown in U.S. Pat. Nos. 3,524,179; 3,729,735; and 4,057,783. In U.S. Pat. No. 3,524,179 distinctive audio tones are produced in response to opening or closing of a switch, a magnetic switching core being employed to sense the transition of a double throw switch. U.S. Pat. No. 3,729,735 describes a control circuit for control of an air conditioner in which the switch closure or opening actuates respective tuning forks to generate respective ultrasonic signals. U.S. Pat. No. 4,057,783 employs an electromagnetic assembly for providing in response to relative rotary movement signals dependent upon switch state and from the phase of which an output signal is derived.